This invention relates to an improved hollow fiber membrane device in which an adjustable, gas permeable, low elastic outerwrap applies a compressive force to the bundle in a direction perpendicular to the bundle's longitudinal axis only during operation of the device.
Membrane devices are used to selectively separate at least one fluid component from a mixture of fluids. Membrane devices are used in a wide variety of separation applications including reverse osmosis and gas separation. Particular gas separations of interest include the recovery of an enriched oxygen stream from air for use in enhanced combustion processes. Alternately, an enriched nitrogen stream may be obtained from air for use as an inert atmosphere over flammable fluids or for food storage. In other embodiments, nitrogen, helium, or carbon dioxide may be separated from hydrocarbons.
Different membrane device configurations suitable for gas separation are described in the art, including plate and frame, tubular, spiral wound, and hollow fiber configurations. The hollow fiber configuration is generally preferred because a higher surface area per unit volume of device can be obtained, resulting in increased device productivity compared to other configurations.
Hollow fiber membrane devices are typically fabricated by parallel or bias wrapping a plurality of hollow fibers about a core. The hollow fibers are embedded in at least one tubesheet and the assembly is inserted into a case. The tubesheet(s) sealingly engages along the inside surface of the case so that two fluid regions are defined, commonly referred to as the shellside and the tubeside regions. The shellside region lies on the outside of the hollow fibers and is defined by the inside wall of the case and the tubesheets which sealingly engage along the inside of wall the case. The tubeside region lies on the inside of the hollow fibers. Communication between the two regions is accomplished by selective permeation of a component(s) through the membrane.
To separate a gas mixture into two portions, one richer and one leaner in at least one component, the mixture is brought into contact with one side of a semipermeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than the other component(s) of the mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component(s) and a stream which is depleted in the selectively permeating component(s). The stream which is depleted in the selectively permeating component(s) is enriched in the relatively nonpermeating component(s). A relatively nonpermeating component permeates more slowly through the membrane than the other component(s). An appropriate membrane material is chosen for the mixture at hand so that some degree of separation of the gas mixture can be achieved.
During operation, the fiber bundle has a tendency to expand when the membrane device is pressurized, resulting in a change in the bundle's packing factor. Packing factor refers to the density of packing of the hollow fibers within the bundle. The packing factor is defined by formula 1: ##EQU1## wherein
N is the total number of hollow fibers in the bundle,
D.sub.o is the outside diameter of the hollow fibers, and
D.sub.b is the diameter of the bundle.
Expansion of the fiber bundle during operation creates nonuniform flow distribution on the shellside of the membrane device which deleteriously affects separation performance. Furthermore, the fibers may be damaged during rapid pressure cycling unless restrained. A means of maintaining a uniform flow distribution during operation and pressure cycling is needed. By applying a uniform compressive force in a direction perpendicular to the longitudinal axis of the bundle, the packing factor for the bundle remains uniform throughout the bundle and uniform flow distribution can be achieved. Uniform compression along the fiber bundle may be accomplished through the use of an outerwrap.
Outerwraps used on membrane devices have been used in the past to protect the fiber bundle from damage during handling or to facilitate insertion of the fiber bundle into a case. Such outerwraps have generally been fabricated from a highly elastomeric material. While highly elastomeric outerwraps apply the necessary uniform compressive force to the fiber bundle, the bundle is under compression at all times. This generally results in loss of performance in devices in which the feed is introduced on the shell side; such performance losses are related to flow distribution problems and compression of the fibers during non-use of the device, among other causes. One such performance loss is a decline in non-permeate flow from the initial performance value due to the compressive force exerted on the bundle during non-use. An outerwrap which applies a uniform compressive force along the fiber bundle only during operation and minimal or no compression during non-use is needed.